[NFC][TLI] Replace std::lower_bound call in getLibFunc with DenseMap lookup

While std::lower_bound takes logarithmic time (relative to the length of
the array) to execute, DenseMap gives better performance characteristics
as it traverses few (if any) elements when collisions do occur,
especially when the number of elements are known in advance.

This gives a speedup of 0.24%:
https://llvm-compile-time-tracker.com/compare.php?from=ac00cca3d9c6c3e9118ebbe47aa5b3ba1ee7404f&to=7f3d4c8ce8cee3a236a2328e46b2a8374672b46e&stat=instructions:u

Differential Revision: https://reviews.llvm.org/D157951

Author: dc03

llvm/lib/Analysis/TargetLibraryInfo.cpp

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Support/CommandLine.h"
@@ -940,16 +941,26 @@ static StringRef sanitizeFunctionName(StringRef funcName) {
   return GlobalValue::dropLLVMManglingEscape(funcName);
 }
 
+static DenseMap<StringRef, LibFunc>
+buildIndexMap(ArrayRef<StringLiteral> StandardNames) {
+  DenseMap<StringRef, LibFunc> Indices;
+  unsigned Idx = 0;
+  Indices.reserve(LibFunc::NumLibFuncs);
+  for (const auto &Func : StandardNames)
+    Indices[Func] = static_cast<LibFunc>(Idx++);
+  return Indices;
+}
+
 bool TargetLibraryInfoImpl::getLibFunc(StringRef funcName, LibFunc &F) const {
   funcName = sanitizeFunctionName(funcName);
   if (funcName.empty())
     return false;
 
-  const auto *Start = std::begin(StandardNames);
-  const auto *End = std::end(StandardNames);
-  const auto *I = std::lower_bound(Start, End, funcName);
-  if (I != End && *I == funcName) {
-    F = (LibFunc)(I - Start);
+  static const DenseMap<StringRef, LibFunc> Indices =
+      buildIndexMap(StandardNames);
+
+  if (auto Loc = Indices.find(funcName); Loc != Indices.end()) {
+    F = Loc->second;
     return true;
   }
   return false;